[Windows] Preserve composing extent in setEditingState - #189968

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@Renzo-Olivares Would you mind being the second reviewer for this change? Matt from Canonical - who normally reviews Windows changes - is out on vacation for the next two weeks. Thanks in advance! :)

@tjcGoogle It looks like the test is failing in CI: https://logs.chromium.org/logs/flutter/buildbucket/cr-buildbucket/8674821850985549569/+/u/test:_Host_Tests_for_host_debug_test/stdout

[----------] 1 test from TextInputPluginTest
[ RUN      ] TextInputPluginTest.SetEditingStatePreservesComposingRange
../../../flutter/shell/platform/windows/text_input_plugin_unittest.cc(543): error: Value of: modifier.GetActiveModel()->composing()
  Actual: false
Expected: true

../../../flutter/shell/platform/windows/text_input_plugin_unittest.cc(544): error: Expected equality of these values:
  modifier.GetActiveModel()->composing_range()
    Which is: 24-byte object <98-DC BB-57 F7-7F 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00>
  TextRange(1, 3)
    Which is: 24-byte object <98-DC BB-57 F7-7F 00-00 01-00 00-00 00-00 00-00 03-00 00-00 00-00 00-00>

[  FAILED  ] TextInputPluginTest.SetEditingStatePreservesComposingRange (14 ms)
[----------] 1 test from TextInputPluginTest (14 ms total)

Follow-up fix for the failing host test: the typo fix alone could not preserve the composing range.

In setEditingState, the handler first calls the single-argument SetText(text), which resets the model composing state (composing_ = !composing_range.collapsed() with a collapsed default TextRange(0)). The subsequent SetComposingRange call then always bails out on its !composing_ guard, so the composing range sent by the framework was silently dropped (model ends with composing() == false, range (0,0) — exactly what the CI failure shows).

This commit applies the text, selection, and composing range in one SetText call (the API text_input_model.h documents for framework editing-state sync), and translates a -1/-1 composing range to 0/0 to match the existing selection handling, which is equivalent to the previous EndComposing() path.

I verified the failure and the fix locally by driving the real TextInputModel through the same call sequence: the old sequence leaves composing()==false / range (0,0), the new sequence preserves [1,3]. Reverting the extent read (back to base) in the new flow makes the regression test fail again on composing(), so the test still guards the original typo. Windows CI will confirm on the real target.

Looks like et format has to be run to fix formatting errors in text_input_plugin_unittest.cc.

autosubmit label was removed for flutter/flutter/189968, because - The status or check suite Google testing has failed. Please fix the issues identified (or deflake) before re-applying this label.